Hydrazines find a variety of uses in silver halide photography. They have been used in negative working surface latent image forming silver halide emulsions to increase speed and/or contrast. They have been used in direct positive internal latent image forming emulsions as nucleating agents.
Hydrazines may also be used for internal latent image direct positive emulsions such as those described in U.S. Pat. No. 3,227,552 and reviewed in Research Disclosure No. 235 (1983) item 23510.
The most efficient hydrazines employed in silver halide photographic systems employ a combination of substituents to balance activity and stability. The stability of hydrazines is increased by attaching directly to one of the nitrogen atoms a tertiary carbon atom, such as the carbon atom of an aromatic ring. The art has long recognized that the activity of these stabilised hydrazines can be advantageously modified by the direct attachment of an acyl group to the remaining nitrogen atom. Thus, the most commonly employed hydrazines are arylhydrazides.
Silver halide emulsions and/or developers containing hydrazines are disclosed, for example, in U.S. Pat. Nos. 2,419,975, 2,563,785, 3,227,552, 3,386,831, 3,730,727, 4,030,925, 4,031,127, 4,080,207, 4,168,977, 4,224,401, 4,243,739, 4,245,037, 4,255,511, 4,266,013, 4,272,614, 4,276,364, 4,323,643, 4,478,928 and 4,560,638 and U.K. Patent Specifications Nos. 1560005, 1579956, 2034908A and 2066492B.
In particular U.S. Pat. No. 2,419,975 discloses that high-contrast negative images are obtained by the addition of hydrazine compounds. This patent describes that extremely high-contrast photographic characteristics, such as a gamma (.gamma.) of more than 10, can be obtained by adding a hydrazine compound to a silver chlorobromide emulsion and developing at a pH as high as 12.8. However, the strongly alkaline developer having a pH of nearly 13 is susceptible to air oxidation and is too unstable to be stored or used for long periods.
U.S. Pat. No. 4,168,977 discloses the use of a hydrazine of the formula: EQU R.sup.1 NHNHCHO
in which R.sup.1 represents an aryl group, in combination with silver chlorobromide or silver chlorobromoiodide emulsions. This combination is capable of functioning at a lower pH than the hydrazines of U.S. Pat. No. 2,419,975 and a pH of 11.5 is exemplified.
U.S. Pat. No. 4,224,401 discloses the use of a hydrazine of the formula: EQU R.sup.1 NHNHCOR.sup.2
in which R.sup.1 is an aryl group and R.sup.2 is a hydrogen atom, C.sub.1 to C.sub.3 alkyl or phenyl optionally substituted with substitutents which are preferably electron withdrawing,
in combination with silver bromide or silver iodobromide emulsions. This combination is also functional at a pH lower than that disclosed in U.S. Pat. No. 2,419,975 and a pH of 11.5 is exemplified when R.sup.2 is hydrogen.
There is a need to provide alternative hydrazines which allow a lowering of pH to reduce adverse effects incurred by aerial oxidation of the developer.
The present invention provides an alternative group of hydrazines which may be employed in silver halide emulsions to provide ultrahigh contrast materials which may be developed at a pH below 12.5.